The present disclosure relates to optical data storage media, and more particularly, to holographic storage mediums as well as methods of making and using the same.
Holographic storage is data storage of holograms, which are images of three dimensional interference patterns created by the intersection of two beams of light, in a photosensitive medium. The superposition of a reference beam and a signal beam, containing digitally encoded data, forms an interference pattern within the volume of the medium resulting in a chemical reaction that changes or modulates the refractive index of the medium. This modulation serves to record as the hologram both the intensity and phase information from the signal. The hologram can later be retrieved by exposing the storage medium to the reference beam alone, which interacts with the stored holographic data to generate a reconstructed signal beam proportional to the initial signal beam used to store the holographic image.
Each hologram may contain anywhere from one to 1×106 or more bits of data. One distinct advantage of holographic storage over surface-based storage formats, including CDs or DVDs, is that a large number of holograms may be stored in an overlapping manner in the same volume of the photosensitive medium using a multiplexing technique, such as by varying the signal and/or reference beam angle, wavelength, or medium position. However, a major impediment towards the realization of holographic storage as a viable technique has been the development of a reliable and economically feasible storage medium.
Early holographic storage media employed inorganic photorefractive crystals, such as doped or undoped lithium niobate (LiNbO3), in which incident light creates refractive index changes. These index changes are due to the photo-induced creation and subsequent trapping of electrons leading to an induced internal electric field that ultimately modifies the index through a linear electro-optic effect. However, LiNbO3 is expensive, exhibits relatively poor efficiency, and requires thick crystals to observe any significant index changes.
More recent work has led to the development of polymers that can sustain larger refractive index changes owing to optically induced polymerization processes. These materials, which are referred to as photopolymers, have significantly improved optical sensitivity and efficiency relative to LiNbO3 and its variants. In prior art processes, “single-chemistry” systems have been employed, wherein the media comprise a homogeneous mixture of at least one photoactive polymerizable liquid monomer or oligomer, an initiator, an inert polymeric filler, and optionally a sensitizer. Since it initially has a large fraction of the mixture in monomeric or oligomeric form, the medium may have a gel-like consistency that necessitates an ultraviolet (UV) curing step to provide form and stability. Unfortunately, the UV curing step may consume a large portion of the photoactive monomer or oligomer, leaving significantly less photoactive monomer or oligomer available for data storage. Furthermore, even under highly controlled curing conditions, the UV curing step may often result in variable degrees of polymerization and, consequently, poor uniformity among media samples.
Other prior examples of holographic recording media are based on “two-chemistry” systems, wherein a binder or material that provides the medium with form and stability, is different from the photoactive component. These systems comprise a mixture of at least one photoactive polymerizable liquid monomer or oligomer, an initiator, at least one precursor (i.e., monomers or oligomers) to the binder polymer, and optionally a sensitizer. These mixtures also initially have a gel-like consistency until the precursors to the binder polymer are partially cured to provide form and stability to the medium. Problems similar to those described for single-chemistry systems may occur during the UV binder cure step. The medium, prior to data storage, has a uniform refractive index based on the weight fraction of each component and their individual refractive indices. Polymerization of the photoactive monomers (or oligomers) leads to the formation of a polymer that has a refractive index different from that of the binder. Photoactive monomer molecules diffuse into the region of polymerization, while binder material diffuses out because it does not participate in the polymerization. Spatial separation of the photopolymer, formed from the monomer, and the binder provides the refractive index modulation required to form a hologram. While better results are obtained using these two-chemistry systems, the possibility exists for reaction between the precursors to the binder polymer and the photoactive monomer. Such reaction would reduce the refractive index contrast between the binder and the polymerized photoactive monomer, thereby affecting any stored holograms. Furthermore, two-chemistry systems may also be plagued by changes in dimension owing to shrinkage induced by polymerization of the photoactive monomers during data recording.
Thus, there remains a need for improved polymer systems suitable for holographic data storage media. It would be advantageous if the binder curing step, which provides stability and form to the media, did not reduce the amount of photoactive material available for data storage. It would be further advantageous if the curing step resulted in consistent levels of binder polymerization between media samples, the possibility for reaction between the binder and the photoactive material were diminished, and dimensional changes during data recording were eliminated.